Newbie here just in first couple of days of initial test setup.
I have Domoticz running on Rpi3 with a Arduino Uno as MySensors gateway via USB (with nrf24 radio). As my initial test setup i have a DHT22 temp+humidity node setup on another Uno with nrf24. My Rpi3 has static ip setup and dynamic dns / port forwarding etc running on my home router.

So i have temp node in bedroom upstairs and rpi/gw downstairs next to my router. When i power everything up, all works perfectly, temp node is showing temp+humidity and updating every minute or so on Domoticz. Remote access even working ok if i drive into town and use my iphone (Pilot app).

Problem ive had, after 2-3hrs when i check Domoticz i get "unable to connect to server, retry", even on home PC (local network). All the LEDs on the MySensors gateway are still lit up and on the radios, if i SSH into my Rpi3, the Pi is still online and working fine. It seems for some reason the Domoticz server on the Pi has stopped and/or crashed ? If i reboot the Rpi3 then all works again fine?

Any ideas please good folks?
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PS: I am sure its possible to have BOTH Domoticz server and the MySensors gateway running on the same Rpi (with radio attached) but ive had no joy getting that to work, hence i used a usb Uno gateway. Any guides on getting it all on the Rpi ?

Thanks Lebo for such a detailed answer, much appreciated.
Strangely it appears to be working fine again currently! Since i made the original post its all been online running for 33hrs without issue, so i shall keep an eye on it.